Halogenated rubbers in particular those comprising repeating units derived from isoolefins are industrially prepared by carbocationic polymerization processes. Of particular importance are chlorobutyl rubber and bromobutyl rubber which are halogenated elastomers of isobutylene and a smaller amount of a multiolefin such as isoprene.
In the conventional process for producing halogenated butyl rubber (also denoted as halobutyl rubber or HIIR) e.g. isobutene and isoprene are first polymerized in a polar halohydrocarbon medium, such as methyl chloride with an aluminum based initiating system, typically either aluminum trichloride (AlCl3) or ethyl aluminum dichloride (EtAlCl2). The butyl rubber does not appreciably dissolve in this polar medium, but is present as suspended particles and so this process is normally referred to as a slurry process. Residual monomers and polymerization medium are then steam stripped from the butyl rubber, before it is dissolved in an organic medium, typically a non-polar medium such as hexane. The halogenation process ultimately produces the final halogenated product in a conventional manner.
After halogenation of butyl rubber the reaction mixture typically comprises the butyl halogenated rubber and the diluent. This mixture which is typically a solution is after neutralization and phase separation typically either batchwise or more commonly in industry continually transferred into a steam-stripper wherein the aqueous phase comprises an anti-agglomerant which for all existing commercial grades today is a fatty acid salt of a multivalent metal ion, in particular either calcium stearate or zinc stearate in order to form and preserve halogenated butyl rubber particles, which are more often referred to as “halobutyl rubber crumb”
The water in this vessel is typically steam heated to remove and recover the diluent.
As a result thereof a slurry of halogenated butyl rubber particles is obtained which is then subjected to dewatering to isolate halogenated butyl rubber particles. The isolated halogenated butyl rubber particles are then dried, baled and packed for delivery.
The anti-agglomerant ensures that in the process steps described above the halogenated butyl rubber particles stay suspended and show a reduced tendency to agglomerate.
In the absence of an anti-agglomerant the naturally high adhesion of halogenated butyl rubber would lead to rapid formation of a non-dispersed mass of rubber in the process water, plugging the process. In addition to particle formation, sufficient anti-agglomerant must be added to delay the natural tendency of the formed halogenated butyl rubber particles to agglomerate during the stripping process, which leads to fouling and plugging of the process.
The anti-agglomerants in particular calcium and zinc stearates function as a physical-mechanical barrier to limit the close contact and adhesion of butyl rubber particles.
The physical properties required of these anti-agglomerants are a very low solubility in water which is typically below 20 mg per liter under standard conditions, sufficient mechanical stability to maintain an effective barrier, and the ability to be later processed and mixed with the butyl rubber to allow finishing and drying.
The fundamental disadvantage of fatty acid salts of a mono- or multivalent metal ion, in particular sodium, potassium calcium or zinc stearate or palmitate is their chemical interaction with rubber cure systems.
Therefore, there is still a need for providing a process for the preparation of halogenated rubber particles in aqueous media having reduced or low tendency of agglomeration.